1. Field of the Invention
The present invention relates to an optical pickup device for a magneto-optical disc reproducing system. More particularly, the present invention relates to an improved optical pickup device for a magneto-optical disc reproducing system for reading out data recorded on a magneto-optical memory medium.
2. Prior Arts
Recently, many efforts have been made for utilizing as a data recording medium a material having varied optical properties in accordance with the magnetic states thereof. As a result thereof, a magneto-optical disc has been developed. The magneto-optical disc has various polarized states of the reflected light in accordance with the magnetic states of the recorded layer of the magneto-optical disc. Using the differences between the polarized states, data can be recorded on a disc and read out from the disc, and a magneto-optical disc wherein such a principle is applied is used as a data memory medium of a large capacitance. In order to record data on the magneto-optical disc and read out the data recorded on the magneto-optical disc, an optical pickup device is formed by integrating a magnetic head and an optical head.
One example of such an optical pickup device is disclosed in U.S. Pat. No. 5,020,041. FIG. 1 is a schematic view for illustrating a magneto-optical memory system having a conventional optical pickup device disclosed in the above U.S. patent.
In the optical pickup device in the magneto-optical reproducing system as shown in FIG. 1, a magneto-optical disc 1 which is rotated by a rotation mechanism such as a rotary motor, includes a magneto-optical recording layer 101 having data recorded by a magneto-optical effect, a disc-shaped transparent substrate 103 towards an optical head 11 and a protection layer 102 towards a magnetic head 12. A light emitted from a light source which is a semiconductor laser 2 is collimated by a collimator lens 3 and directed to a focusing lens 5 mounted on an actuator 6 through a beam splitter 4. The light beam focused by focusing lens 5 is directed to the substrate 103 of the disc to thereby form a fine spot of approximately 1 .mu.m in diameter on recording layer 101.
The focused light beam is polarized in accordance with the magnetic state of recording layer 101 and then reflected. The reflected light passes through focusing lens again and is reflected by beam splitter 4, and then is directed to a magneto-optical-signal detecting optical system 8 and a light point control signal detecting optical system 9 for detecting defocusing and off-track, by a beam splitter 7. Magneto-optical signal detecting system 8 is a differential signal detection system which is comprised of a .lambda./2 plate 801 and a polarization beam splitter 803. A light applied to magneto-optical signal detecting optical system 8 passes through .lambda./2 plate 801 and a lens 802, end is separated into two polarization components (S and P polarization components) by polarization beam splitter 803. The polarization components are detected by photo-detectors 804 and 805, respectively, and converted to electrical signals, which are differentiated by a differential amplifier 10, which in turn produces a magneto-optical signal.
A floating magnetic head 12 is arranged on the recording layer side which is on the opposite side to the optical head 11 with respect to disc 1. Floating magnetic head comprises a coil for applying a magnetic field to recording layer 101 and a slider for floating the head assembly and it floats by an air pressure created by the disk rotation while the disc rotates. The floating magnetic head 12 is integrally coupled to the optical head 11 by a support arm 15 so that it moves with the optical head 11 and the distance between both heads is kept constant.
Optical head 11 in the conventional optical pickup device as shown in FIG. 1 comprises a semiconductor laser 2 for generating a light, collimator lens 3 for collimating the light, beam splitter 4, focusing lens 5 for focusing the light beam passed through beam splitter 4 to form a fine spot on the disc recording layer, an actuator 6 for driving the focusing lens, beam splitter 7 for splitting the reflected light from disc 1 and directing this to the light point control signal detecting optical system 9 and magneto-optical signal detecting optical system 8 which comprises .lambda./2 plate 801, lens 802, a polarization beam splitter 803 and photo-detectors 804 and 805. Since the above conventional optical pickup head comprises a large number of members, it has a very complicated structure and has difficulties in maintaining precise positions between those members.
Another example of an optical pickup device is disclosed in U.S. Pat. No. 5,157,649 (issued to Masayuki Suzuki). FIG. 2 is a schematic view for illustrating an optical device disclosed in the above U.S. patent. The shown optical pickup device includes a collimator lens 33 for changing a light from a light source 31 into a parallel beam, a beam shaping prism 34 for shaping an elliptical shape of the parallel beam into a circular shape in cross section, a half-wave plate 35 for rotating the plane of polarization of the parallel beam having the circular shape by 90 degrees, a splitting plane 36A for vertically reflecting a light which has passed half-wave plate 35 with a radial direction of a disc, a beam directing prism 37 for directing the light reflected from splitting plane 36A to an objective lens 38, a half-wave plate 40 through which the reflected light from disc which has passed the objective lens 38, the beam directing prism 37 and the splitting plane 36A, a total reflection prism for directing the light which has passed half-wave plate 40 so as to be perpendicular to the radial direction of the disc, a polarization beam splitter 41 for splitting the directed beam into two beams, and two photo sensors 45 and 46 for sensing the two beams split by the polarization beam splitter respectively by using two convex lenses 42, a total reflection prism 44 and a Cylindrical lens 43.
The optical pickup device having the above structure does not having a protruding portion in the radial direction of the disc and therefore the overwhole size of the disc player may be compact. However, the structure of the disc player is very complicated and thus the optical head thereof becomes large. Also, there are many difficulties in controlling the precise positions of the components when manufacturing the disc player. This increases the manufacturing cost and the reliability of the disc player can not be ensured due to the difficult manufacturing. Therefore, the techniques for manufacturing a disc player having a simple structure which increases the reliability thereof and reduces the manufacturing costs have been required.